In recent years, there has been a rapid progress in the research and development of a high-density optical disc system capable of recording/reproduction of information using a blue-violet semiconductor laser having a wavelength of 400 nm. For example, a so-called Blue-ray Disc (BD), an optical disc for recording/reproduction of information based on the specification with a NA of 0.85 and a light source wavelength of 405 nm, which permits 20 through 30 GB information to be recorded on an 12 cm-diameter optical disc having the same size as a DVD (having a NA of 0.6, a light source wavelength of 650 nm and a storage capacity of 4.7 GB). A HD DVD, an optical disc for recording/reproduction of information based on the specification with a NA of 0.65 and a light source wavelength of 405 nm, permits 15 through 20 GB information to be recorded on an 12 cm-diameter optical disc. In the present specification, such an optical disc will be referred to as a “high-density optical disc”.
It must be noticed that the sufficient value of an optical pickup apparatus as a commercial product is not provided by mere capability of adequate recording/reproduction of information using such a high-density optical disc. At present, the DVD and CD containing a great variety of information are displayed on the market. In view of this situation, mere capability of adequate recording/reproduction of information using a high-density DVD cannot meet the requirements of the market. For example, to provide capability of recording/reproducing the information using the prior art DVD or CD owned by a user is to improve the value of the commercial product as a compatible type optical pickup apparatus. Against this background, the optical system using a compatible type optical pickup apparatus is-required to provide adequate recording/reproduction of information using any of the high-density DVD, prior art DVD and CD, in addition to the advantages of low cost and simple structure. Further, an optical pickup apparatus capable of providing adequate recording/reproduction of information using the DVD and CD in a compatible mode has come into practical use. However, the current products are required to be reformed into downsized and low-profile versions.
Of the high-density optical discs, the BD uses a protective layer having a thickness of about 0.1 mm (“protective layer” in the sense in which it is used here refers to the transparent layer arranged on the side where the light beams are applied to the information recorded surface of the optical disc; also called “transparent substrate or protective layer”). This is intended to reduce the coma aberration resulting from the curvature of the optical disc and inclination when the objective lens uses a higher numerical aperture. However, this produces a big difference in the thickness of the protective layer between the CD (protective layer: 1.2 mm thick) and DVD (protective layer: 0.6 mm thick). Thus, it is necessary to ensure compatibility among these optical discs of various specifications, by using a common objective lens, without causing a substantial increase in costs. In this case, when using any type of the optical disc, the value of a product is higher if the wait time is shorter, the wait time being the time required to reach the enable status of recording/reproduction function, subsequent to loading an optical disc into the drive apparatus. Accordingly, the optical pickup apparatus is required to provide a high-speed compatibility function.
Another problem is that the adequate recording/reproduction of information is discouraged by spherical aberration caused by the difference in the thickness of protective layer. This spherical aberration occurs in proportion to the fourth power of the numerical aperture of the objective lens. In the case of a prior art CD or DVD having a smaller numerical aperture, the spherical aberration resulting from the thickness of the protective layer is sufficiently small. There has been no need of taking a special procedure for correcting the spherical aberration. By contrast, when the numerical aperture of the objective lens is as high as 0.85, for example, the tolerance in the thickness of the protective layer is very close on the order of several microns. When consideration is given to residual spherical aberration of the optical pickup apparatus and the volume productivity of the optical disc, the optical pickup apparatus is preferably provided with a spherical aberration correcting function. Further, when the error in the thickness of the protective layer of the optical disc has to be corrected on a real-time basis, a means for correcting spherical aberration is required to provide a high degree of responsibility.
In addition, when an objective lens is created using an objective lens subjected to a change in refraction index depending on temperature change, spherical aberration will deteriorate in response to the temperature change. This spherical aberration must also be corrected. In an optical pickup apparatus permitting recording/reproduction of information using different types of optical discs on a compatible basis, the aforementioned problems must be solved, and a compact and low-cost product must be provided.
Further, what is known in the prior art includes the so-called two-layer DVD where the storage capacity is almost doubled by alternate lamination of the protective layer and information recording layer from the same light beam entry side. Especially the next-generation optical disc system utilizes the objective lens of higher numerical aperture than that of the DVD. If recording/reproduction of information is intended using such a dual layered recording type optical disc, there is a big difference resulting from the difference in thickness in the range from the light beam entry side to each of the information recording layers, at the time of focus jumping between the information recording layers. Accordingly, in the next-generation optical disc system, the focusing of the objective lens as well as correction of the spherical aberration are essential at the time of focus jumping between information recording layers.
To solve the aforementioned problems, a condensing optical system has been proposed. This condensing optical system has a beam expander composed of positive and negative lenses, and one of these lenses is movable along the optical axis. However, the optical pickup apparatus is often installed in a very limited space. The problem with the optical pickup apparatus is how to drive the lens as a constituent of the beam expander, which provides a drive means. The Official Gazette of Japanese Patent Tokkai 2001-28147 discloses a structure of driving a combination lens using a voice coil motor. In the meantime, the Official Gazette of Japanese Patent Tokkai 2003-45068 discloses a structure of driving the relay lens using a stepping motor.
In this connection, the high-density optical disc, DVD and CD use different light source wavelengths, and a common light source cannot be utilized. Accordingly, in an optical pickup apparatus of compatible type, a light source must be arranged for each of them. By contrast, a so-called 2-laser 1-package has come on the market. In this system, two light emission points for emitting the light beams having different wavelengths are fixed in one heatsink. It is known that use of this system allows a compact configuration of the optical pickup apparatus.
However, the light emission points of this 2-laser 1-package are apart from each other at a distance of about 100 μm. When they are used as laser light sources of the optical pickup apparatus, if either of the light sources is adjusted to the reference optical axis of the optical pickup apparatus, another light source will be off the reference optical axis. When a laser beam is emitted from the emission point off the reference optical axis, a distance is created between the optical axis and the image point on the image surface. Especially in the objective lens for emitting the divergent light, a great amount of deterioration is caused by off-center characteristics. For example, coma aberration tends to occur at the time of tracking. Suck a problem is left unsolved.
Further, when the optical disc is curved due to heat and others, and is tilted with respect to the surface orthogonal to the optical axis, coma aberration also occurs to the optical spot. When the objective lens actuator of poor tilt characteristics is employed, the objective lens may tilt at the time of focusing and tracking. If the objective lens tilts, coma aberration tends to occur in the optical spot for an objective lens with priority placed on distance between the optical axis and the image point (design impervious to lens shift). Suck a problem is left unsolved.
To meet the high-speed rotation of the recording type DVD, efforts have been made to increase the laser power. The high-power laser is generally characterized by a higher threshold value. If an attempt is made to use it for regeneration as well and to apply the lower-power laser beam, noise tends to increase. In this manner, handling of the laser light source is anticipated to become difficult in future. In case of a high power laser meeting the current rotational speed, there is no special problem when used with the power controlled, even at the time of low power for DVD regeneration. A 2-laser 1-package type laser light source equipped with a high power laser for recording and a low power laser for scanning is expected to come on the market to provide measures for solving the aforementioned problem anticipated when laser power is further increased in future. However, the problem is how to control the coma aberration caused by deviation of the light source from the axis in such a laser light source. In the aforementioned beam expander, the coma aberration cannot be corrected. This makes it necessary to install an independent structure for correcting coma aberration, and hence raises a problem of an increased size of the optical pickup apparatus.
In the meantime, it is known that coma aberration can be corrected by shifting the optical device in the direction orthogonal to optical axis. When the optical device is to be shifted in the direction orthogonal to optical axis for the purpose of correcting the coma aberration, a voice coil motor disclosed in the Official Gazette of Japanese Patent Tokkai 2001-28147 can be used as a drive means. This will provide the benefit of increased response speed. However, the voice coil motor is intended to move a movable section in the form of a circular arc. When the optical device mounted on the movable section is moved a considerable distance in the direction orthogonal to the optical axis, it may be moved in the unintended direction of optical axis. This may cause deterioration of the spherical aberration. Further, for the very nature of the voice coil motor, current must be applied continuously to hold the optical device at the home position. This will raise the problem of increased power consumption. This problem tends to occur especially in a battery-driven apparatus such as a notebook PC. Another problem is found in the high cost of the voice coil motor.
On the other hand, when the stepping motor disclosed in the Official Gazette of Japanese Patent Tokkai 2003-45068 is used as a drive means, the movable section can be shifted in a straight line, and the cost is comparatively low. However, when one wishes to get high resolution required for the control of the optical pickup apparatus, the response speed will be reduced. Further, the stepping motor involves a vibration problem and loss of synchronism wherein conformance to input pulses fails when torque for the load is insufficient. Generally, the stepping motor is large-sized, and hence the installation site will be limited.